64 NEW YORK STATE MUSEUM 



tiny zircons do not fail. Quartz is not entirely lacking, but in 

 typical specimens it is a minor component. The syenite was called 

 in the writer's earlier paper, the " Barton gneiss." A number of 

 analyses have been made for Professor Cushing from specimens 

 gathered in the northern Adirondacks and they uniformly run 

 below 65 per cent Si0 2 , the percentage at which quartz begins to 

 be an important mineral in the eruptive rocks. 



In the cores as well as in the hand specimens the syenite is a 

 blotchy, black and green rock, which always has a pronounced 

 green cast when fresh. On ledges that have been long exposed to 

 weathering it is often decidedly rusty, especially in the basic 

 phases. While the percentage in iron is not so very high, yet this 

 element must be combined in one or more of the minerals in some 

 unstable form, such that it readily oxidizes. It is often necessary 

 to break into good sized blocks before the reasonably fresh green 

 rock appears at the core. It has also been our experience in the 

 field to find the syenite sometimes developing on exposure a dead 

 white crust that resembles the anorthosites and that is deceptive. 

 In these varieties the iron must be in small amount or else limited 

 to some stable compound that resists decay. In fact with the 

 variations to be next outlined and the protean appearance on 

 weathering, it is not surprising that the syenitic rocks have so long 

 escaped identification as such. 



As a departure from the normal proportions of feldspar and 

 dark silicate, we sometimes find the latter developing in greatly 

 increased amount. The feldspar is far less prominent and a dark 

 basic rock ensues which on slight acquaintance one would consider 

 a basic gabbro or diorite. But the characteristic feldspar, as well 

 as the normal dark silicates of the syenite, are still present, and both 

 in the drill cores, as well as in the field, we find a quick passage from 

 the usual variety to the basic with no eruptive contact that would 

 indicate a separate intrusive mass or an included sediment. For 

 these dark bands we can adduce no other reasonable conception 

 than that the original intrusive mass was in parts more basic than 

 elsewhere, and that if its parent magma were homogeneous, it 

 separated, as has been so often observed in later years in large 

 eruptive masses, into portions of contrasted composition although 

 of common parentage. This basic syenite was not recognized as 

 such in the writer's previous paper, but was esteemed to be a 

 gneissoid representative of the gabbro. While it resembles this 

 rock in the closest way, yet the drill cores now available prove 

 its affinity with the syenites. 



